The present invention relates to effective technique applied to an imaging system using a solid-state CMOS imaging device and more particularly to effective technique utilized to cancel flicker of a fluorescent lamp in a camera system including automatic iris adjustment function for controlling brightness of an imaged picture in accordance with photographic surroundings.
As an imaging device for a video camera or an electronic still camera, there are a solid-state CCD imaging device and a solid-state CMOS imaging device. The solid-state CMOS imaging device has merits that power consumption thereof is smaller as compared with the solid-state CCD imaging device and it is suitable for compactness and lightness in weight of a video camera and a digital camera. In the solid-state CCD imaging device, electric charges photoelectrically converted and stored for each pixel are transferred to a CCD for transfer simultaneously in parallel at the same timing for all pixels and then transferred in series within the CCD to be outputted, while it is necessary to produce a high potential difference in order to enhance the charge transfer efficiency in the CCD. Consequently, the power consumption thereof is increased.
On the other hand, in the solid-state CMOS imaging device, electric charges photoelectrically converted and stored for each pixel are converted into voltages for respective pixel to be amplified and the amplified voltage for each pixel is selected by a matrix selection circuit successively to be outputted. With such a system, the device can be operated only by a single power supply of about +3.3 volts, for example, and the power consumption thereof can be reduced to one over several as compared with the CCD type. Further, since the solid-state CMOS imaging device can utilize the CMOS process to be fabricated, peripheral circuits such as A-D converters and amplifiers are also apt to be integrated together with the device.
With the above advantages, in recent years, a camera system using the solid-state CMOS imaging device is being used in an application having a particularly higher demand for compactness and lightness in weight such as, for example, a portable data terminal. The solid-state CMOS imaging device is also being used even in a video camera system of various systems including the national television system committee (NTSC) system. In response to such tendency, the solid-state CMOS imaging device with electronic shutter function which includes integrated peripheral circuits such as A-D converters and amplifiers and can set up an electric charge storage time for each pixel externally in each frame is being offered to the market.
When the camera system using the solid-state CMOS imaging device is used under the lighting of a fluorescent lamp in a home or an office, light and dark spots (or difference in brightness) in the form of belt are produced in a frame (picture) of an imaged picture. The light and dark spots are a phenomenon caused by beat interference between a charge storage timing of the solid-state CMOS imaging device and a turning-on-and-off period of a fluorescent lamp and is flicker of the fluorescent lamp of a kind.
The flicker due to the fluorescent lamp is also produced in the camera system using the solid-state CCD imaging device, although since electric charges for all pixels are stored at the same timing in the case of the solid-state CCD imaging device, influence due to the flicker of the fluorescent lamp appears as variation in light and shade (or variation in brightness) among frames and the light and dark spots are not produced in a frame. There is provided a technique for correcting variation in light and shade among frames in the solid-state CCD imaging device by means of, for example, automatic gain control (AGC) relatively easily (for example, JP-A-4-94273 laid-open on Mar. 26, 1992 and JP-A-4-135382 laid-open on May 8, 1992).
However, in a camera system using the solid-state CMOS imaging device, as shown in FIG. 5, it is ascertained that influence due to the flicker of the fluorescent lamp appears as light and dark spots in a striped pattern and the picture quality is deteriorated remarkably. The light and dark spots are produced as follows.
As shown in FIG. 4a, an amount of light of a fluorescent lamp is varied in accordance with twice as high as a frequency of a power supply, that is, a half of a period of the power supply. The frequency of the power supply is generally 50 or 60 Hz and the light amount is varied at the period of 10 milliseconds for 50 Hz and at the period of 8.3 milliseconds for 60 Hz.
The CMOS imaging device is also provided with an electronic shutter function for controlling the charge storage time for each pixel in each frame similarly to the solid-state CCD imaging device. However, start and end timings for storage of electric charges are set up so that storage of electric charges for all pixels in a frame is started and ended at the same timing simultaneously in the solid-state CCD imaging device, whereas storage control of electric charges are made so that the same charge storage time is given to all pixels but its timing is different for each pixel in the solid-state CMOS imaging device.
As described above, in the solid-state CCD imaging device, the electric charges photoelectrically converted and stored for each pixel are transferred simultaneously in parallel at the same timing for all pixels and then read out, while in the solid-state CMOS imaging device electric charges photoelectrically converted and stored for each pixel are successively selected for each pixel by a matrix selection circuit and read out. Accordingly, the charge storage timing of the pixel depends on the reading and selection timing by the matrix selection circuit. That is, the stored electric charges are read out at the selected and read timing by the matrix selection circuit. Accordingly, the charge storage timing for each pixel in the CMOSS type solid-state imaging device is different little by little in order of reading out the pixel.
For example, as shown in FIG. 4b, when the charge storage time for each pixel is T seconds (T<half period of a power supply) for one frame, electric charges for two pixels A and B are stored during the same T seconds while the start and end timings thereof are different for the pixels A and B. Accordingly, even when the charge storage times for the pixels A and B are the same, the stored light amounts (corresponding to areas hatched in FIG. 4B) from the start to the end of the storage of electric charges during the T seconds are different depending on variation in an amount of light of a fluorescent lamp.
Consequently, even when the charge storage time for each pixel is the same, the stored light amount for each pixel is varied and its variation is stored successively to produce light and dark spots (difference in brightness) in a frame. In the case of a video camera of a raster scanning system such as the NTSC, the density of horizontal scanning lines is varied periodically to produce light and dark spots in the form of belt. The light and dark spots produced in the frame are difficult to distinguish from contents of a picture and cannot be corrected by a conventional technique such as AGC.
Heretofore, in order to remove the above disadvantages, there is proposed a system in which a photodiode dedicated to detect flicker is provided within a solid-state CMOS imaging device and a mechanism for reading out a detection signal of the photodiode to judge it in synchronism with a vertical transfer clock differently from reading operation of a signal from a pixel in an picture outputting area is provided so that variation in brightness is sampled to thereby automatically judge the turning-on-and-off frequency of 100 or 120 Hz of the fluorescent lamp and cancel flicker (ISSCC 2001/Feb. 5, 2001, “DIGEST OF TECHNICAL PAPERS” pp. 90–91).
In the system for automatically judging the turning-on-and-off frequency of 100 or 120 Hz of the fluorescent lamp by means of the above method, however, the flicker detection area (photodiode) is provided within the imaging device and accordingly a chip size is increased. Further, since the signal processing circuit dedicated to process the detection signal from the photodiode is required, the chip size is further increased and an increased cost is avoided.